Electrostatic loudspeakers



Dec. 8, 1964 N, zSON GUSSING 3,160,715

ELECTROSTATIC LOUDSPEAKERS Filed Nov. 14, 1961 4 Sheets-Sheet l FIG.1 FI G.2

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E ECTRODE POTENTlAL ELECTRODE 1"" ELECTRODE 2 DISTANCE Arryi 1964 N. T. AISON GUSSING 3,150,715

ELECTROSTATIC LOUDSPEAKERS 4 Sheets-Sheet 2 Filed Nov. 14, 1961 FIGS 2 E D 0 R T C E L E ELECTRODE 1 1 I 1 cl DISTANCE POSITION FOR DIAPHRAGM 1964 N. T. A SON GUSSING 3,160,715

ELECTROSTATIC LOUDSPEAKERS Filed Nov. 14, 1961 4 Sheets-Sheet 3 FIG.6cl

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OPERATION RANGE l I I {/ELECTRODE 2 I l 8, 1964 N. T. A SON GUSSING 3,160,715

ELECTROSTATIC LOUDSPEAKERS 4 Sheets-Sheet 4 Filed Nov. 14, 1961 POTENTIAL CURVE AT CONTINUOUS BALANCE CENTERING FORCE E v R U C L m T N E 0 P AT CERTAIN OPERATION RANGE /'O. ELECTRODE 2 T ELECTRODE 1 LOU D- SPEAKER i AMPLI' FlER FIG.8

MODU W LATOR INVER- \J SIGNAL TER OSC ILLA' TOR United States Patent ELECTRQSTATIC LQUDSPEAKERS Nils Torsten Amen Gnssing, Soistigen 13, Saltsiohaden, Sweden Filed Nov. 14, 1951, Ser. No. 152,235 Claims priority, application Sweden, Nov. 15, 1960, 1%,972/ 6t) 8 Ciaims. (Cl. 17--111) This invention relates to a device for stable suspension of a freely movable electrically conductive surface between two fixed electrodes and more particularly to electrostatic loudspeakers.

Those electrostaitc loudspeakers which now are in use, are normally equipped with a moving diaphragm of electrically conducting material and with two fixed perforated electrodes arranged in parallel with the diaphragm. The diaphragm and the fixed electrodes are given a constant electric charge on which is superimposed the alternating voltage, which is to be reproduced acoustically. This construction has several drawbacks. The electromechanical laws for this type of loudspeaker give a nonlinear action of the diaphragm, which will be noticed as a severe distortion at large amplitudes. The diaphragm is always in a nonstable position, because the attractions between the diaphragm and the electrodes are inversely proportional to the square of the distance between the diaphragm and the electrodes, which must be counteracted by a rigid suspension of the diaphragm. The useful amplitude will be diminished by both these factors, so that the bass reproduction will be poor. A third drawback is the heavy reactive load that the loudspeaker represents to the amplifier, which causes matching difficulties. Finally the loudspeaker is to be given a high biasing D.C.-voltage in the order of kilovolts.

The following requirements may be established for an ideal electrostatic loudspeaker. V

(1) The diaphragm ought to be stabilizer centrally be tween the fixed electrodes by means of electrostatic forces.

(2) The forces acting on the diaphragm, must be proportional within large amplitude limits to the signal or A.C.-voltage, which is to be reproduced.

(3) It must be possible to match the impedance of the loudspeaker to that of the amplifier over the whole ac coustic frequency range.

The main object of the invention is to provide an elec trostatic loudspeaker meeting these requirements.

Other objects of the invention and the principle of the loudspeaker will be disclosed in the following description with reference to the accompanying drawings, in which:

FIGS. 1 and 2 show schematically the principle of the loudspeaker.

FIG. 3 shows graphically the potential of the electrodes as a function of the distance of the diaphragm from the balance position.

FIG. 4 shows a circuit diagram for a loudspeaker according to the invention.

FIG. 5 shows graphically the impedances of the parallel resonant circuits L-C and L 0 respectively, in FIG. 4 for dilferent positions of the diaphragm.

FIGS. 6a and 6b show a circuit diagram of another embodiment of the loudspeaker according to the inven tion and the corresponding voltage diagram respectively.

FIG. 7 shows another form of the voltage curves providing a centering force for the diaphragm.

FIG. 8 shows schematically and partially in block form a system including a loudspeaker according to the invention.

Referring first to FIG. 1, there is between the two fixed, perforated electrodes 1 and 2 a conductive diaphragm 3 suspended between flexible mountings 4, which allow "ice large amplitudes of displacement of the diaphragm. The fixed electrodes 1 and 2 are mounted at a distance of 2a from each other, which must be several times larger than the maximum excursion of the diaphragm. If the diaphragm is grounded and the potentials V and V are applied to the fixed electrodes 1 and 2, then the resulting force F actuating on the diaphragm 3, being displaced by the distance x from the midpoint or normal position, will be (see FIG. 2).

The resulting force F will be zero if:

where and v and v being the potentials of the signals to be reproduced.

The force will then be U (CL+QI) 2 i z 2 a (a+x) a (aac) c0nst ugh}1 v2] If the potentials v and v are arranged to be a signal in push-pull modulating a carrier with the unmodulated amplitude V the following relations are given:

v =V [1+ m sin wt] z=Vo[l+m sin (wt-lm=modulation factor F const and the force is F=c0nst V [(l+m sin t) (1- sin wt) Zm sin at] 1 l const E V 4m S111 wt const E V V m S111 wt As can be seen, the force F is proportional to the amplitude V of the carrier and to the modulating signal V .m.sin wt.

An electrostatic loudspeaker working on the said prin- I ciple is constructed as follows. (See FIG. 4). Between the grounded diaphragm 3 and each of the fixed electrodes 1 and 2 are connected two identical inductances L which constitute two separate parallel-resonant tuned circuits L C and L C with the capacitances C and C of electrodes 1 and 2. The frequencies of resonance of these two tuned circuits will vary depending on the position of the diaphragm 3 and will be equal when the diaphragm is in the midpoint (balance) position. If an with a fixed frequency slightly higher than (the midpoint frequency of L C is induced in the two tuned circuits, the amplitude of the induced will Vary with the pOsition of the diaphragm. It will go higher when the distance between the diaphragm and the corresponding electrode is increased and go lower when the distance is decreased. This is shown in FIG. 5, which shows an in the two circuits at varying diaphragm position. Through suitable choice of frequency and circuit-Q it is possible to obtain in the region around the midposition a potential variation that is proportional to the distance between the diaphragm and the corresponding electrode, which earlier was shown to be necessary for an ideal electrostatic loudspeaker. Through parallel connection to C and C of equal capacitance of suitable value, the useful region of the diaphragm movement can be extended (see FIG. 6). This is efiected in practice by a form of construction wherein one entire loudspeaker system is built into a. case or housing of conducting material with perforated walls, the walls being parallel to the fixed electrodes at a distance that gives the desired capacitance. The case is connected to the diaphragm which is grounded. This construction gives the advantage of screening the loudspeaker system, preventing radiation from the high frequency field of the energized electrodes and thus avoiding interference.

Through suitable choice of capacitances, the Q-value of the circuits and the frequency of the carrier it is possible to obtain a working point on the resonance curve (see FIG. 7) where the potential is changing slightly more than proportionally to the distance between the diaphragm and the electrodes, causing the resulting force from carriers always to urge the driver diaphragm to its normal position between the fixed electrodes. In this way the diaphragm :is stabilized between the electrodes, which earlier was said to be imperative. If the carriers then are modulated in push-pull by a signal in known manner, the diaphragm will be {forced to move according to that signal. A suitable system is shown in FIG. 8.

What I claim is: V

1. An electrostatic loudspeaker system comprising: an electrically driven diaphragm member; two acoustically permeable fixed electrode members symmetrically disposed on opposite sides of said driven member; means supporting said driven member for substantially free displacement through a predetermined maximum excursion between said electrode members, said driven member having a normal position within said excursion wherein both electrostatic capacitances between said two fixed electrode members and said driven member are equal; an electrically insulative acoustic fluid medium surrounding all of said members and filling the spaces therebetween; two inductive circuit elements of equal inductances each individually connected to be tuned by one of said capacitances, one of said inductive elements being tuned for resonance at a predetermined frequency with said driven member displaced by a predetermined distance in one direction from said normal position and the other inductive element being tuned for resonance at said predetermined frequency with said driven member displaced by said predetermined distance in the opposite direction from said normal position, oppositely sloping rectilinear portions of the resonance curves of said tuned inductive elements intersecting symmetrically with said driven member in said normal position; and circuit means to energize said inductive elements to cause displacements of said driven member from said normal position, the rectilinearity of said oppositely sloping portions of said resonance curves causing the magnitudes of said displacements to beprecisely proportional to the magnitudes of the energization potentials of said circuit elements.

2. An electrostatic loudspeaker system comprising: a grounded electrically conductive driven diaphragm memher; a pair of spaced acoustically permeable fixed electrode members symmetrically disposed at opposite sides of said driven member; means supporting said driven member for free displacement through a predetermined maximum excursion centrally intermediate said electrode members, said driven member having a normal position within said excursion wherein both electrostatic capacitances between said two fixed electrode members and said driven member are equal; two inductive circuit elements of equal inductances each individually connected between one of said electrode members and said driven member to form two circuits each tuned by one of said capacitances, both circuits being tuned symmetrically off resonance at a fixed predetermined frequency with said driven member in said normal position, oppositely sloping rectilinear portions of the impedance curves of said tuned circuits, when plotted against the position of said driven member, intersecting symmetrically when said driven member is in said normal position; and a push-pull energizing circuit with an effectively grounded midpoint symmetrically connected to said tuned circuits.

3. A system according to claim 2, wherein said energizing circuit comprises a source of said predetermined frequency; two separate modulator means each connected between said source and one of said tuned circuits and input means including phase inversion means adapted to receive a signal to be acoustically reproduced by said driven member, said input means applying said signal to be reproduced in push-pull relationship to said modulator means.

4. A system according to claim 2, wherein the slopes of said impedance curves are dimensioned to cause said driven member to be yieldingly urged toward said normal position when the signals applied to said tuned circuits are equal and balanced with respect to ground.

5. An electrostatic loudspeaker system comprising: a grounded electrically conductive driven diaphragm member; a pair of spaced acoustically permeable fixed electrode members symmetrically disposed at opposite sides of said driven member; means supporting said driven member for free displacement through a predetermined maximum excursion centrally intermediate said electrode members, said driven member having a normal position within said excursion wherein both electrostatic capacitances between said two fixed electrode members and said driven member are equal; two inductive circuit elements of equal inductances each individually connected between one of said electrode members and said driven member to form two circuits each tuned by one of said capacitances, both circuits being tuned symmetrically oif resonance at a fixed predetermined frequency with said driven member in said normal position, oppositely sloping rectilinear portions of the impedance curves of said tuned circuits, when plotted against the position of said driven member, intersecting symmetrically when said driven member is in said normal position; acoustically permeable grounded shielding means defining two further electrode members symmetrically positioned exteriorly of said fixed electrode members and in proximity thereto to provide equal fixed shunt capacitances in parallel with said inductive circuit elements whereby the range of rectilinearitylof said impedance curves is extended in both directions away from the point where said curves intersect; and a push-pull energizing circuit balanced with respect to ground symmetrically connected to said tuned circuits.

6. A system according to claim 5, wherein said energizing'circuit comprises a source of said predetermined frequency; two separate modulator means each connected between said source and one of said tuned circuits; and input means including phase inversion means adapted to receive a signal to be acoustically reproduced by said driven member, said input means applying said signal to be reproduced in push-pull relationship to said modula- References Cited by the Examiner tor means.

7. A system according to claim 5; wherein the slopes UNITED STATES PATENTS of said impedance curves are dimensioned to cause said 1,399,877 12/21 PuPm driven member to be yieldingly urged toward said normal 5 1,762,981 6/30 Hafiley 179111 position when the signals applied to said tuned circuits 19,78,200 10/34 Helsmg 179-411 are equal and balanced with respect to ground. 1,983,377 12/34 Kellogg 179111 v 8. A system according to claim 5, wherein said grounded shielding means forms a housing enclosing said ROBERT ROSE Pnmary Exammer' first-named electrode members and said driven member. 10 WILLIAM C. COOPER, Examiner. 

1. AN ELECTROSTATIC LOUDSPEAKER SYSTEM COMPRISING: AN ELECTRICALLY DRIVEN DIAPHRAGM MEMBER; TWO ACOUSTICALLY PERMEABLE FIXED ELECTRODE MEMBERS SYMMETRICALLY DISPOSED ON OPPOSITE SIDES OF SAID DRIVEN MEMBER; MEANS SUPPORTING SAID DRIVEN MEMBER FOR SUBSTANTIALLY FREE DISPLACEMENT THROUGH A PREDETERMINED MAXIMUM EXCURSION BETWEEN SAID ELECTRODE MEMBERS, SAID DRIVEN MEMBER HAVING A NORMAL POSITION WITHIN SAID EXCURSION WHEREIN BOTH ELECTROSTATIC CAPACITANCES BETWEEN SAID TWO FIXED ELECTRODE MEMBERS AND SAID DRIVEN MEMBER ARE EQUAL; AN ELECTRICALLY INSULATIVE ACOUSTIC FLUID MEDIUM SURROUNDING ALL OF SAID MEMBERS AND FILLING THE SPACES THEREBETWEEN; TWO INDUCTIVE CIRCUIT ELEMENTS OF EQUAL INDUCTANCES EACH INDIVIDUALLY CONNECTED TO BE TUNED BY ONE OF SAID CAPACITANCES, ONE OF SAID INDUCTIVE ELEMENTS BEING TUNED FOR RESONANCE AT A PREDETERMINED FREQUENCY WITH SAID DRIVEN MEMBER DISPLACED BY A PREDETERMINED DISTANCE IN ONE DIRECTION FROM SAID NORMAL POSITION AND THE OTHER INDUCTIVE ELEMENT BEING TUNED FOR RESONANCE AT SAID PREDETERMINED FREQUENCY WITH SAID DRIVEN MEMBER DISPLACED BY SAID PREDETERMINED DISTANCE IN THE OPPOSITE DIRECTION FROM SAID NORMAL POSITION, OPPOSITELY SLOPING RECTILINEAR PORTIONS OF THE RESONANCE CURVES OF SAID TUNED INDUCTIVE ELEMENTS INTERSECTING SYMMETRICALLY WITH SAID DRIVEN MEMBER IN SAID NORMAL POSITION; AND CIRCUIT MEANS TO ENERGIZE SAID INDUCTIVE ELEMENTS TO CAUSE DISPLACEMENTS OF SAID DRIVEN MEMBER FROM SAID NORMAL POSITION, THE RECTILINEARITY OF SAID OPPOSITELY SLOPING PORTIONS OF SAID RESONANCE CURVES CAUSING THE MAGNITUDES OF SAID DISPLACEMENTS TO BE PRECISELY PROPORTIONAL TO THE MAGNITUDES OF THE ENERGIZATION POTENTIALS OF SAID ELEMENT. 